The present invention is directed to a chrome oxide refractory material having improved resistance to attack by silica slags than the prior spinel-containing co-fused chrome oxide refractory materials.
Fused cast refractories are well known in the art. Such refractories which are generally based upon a spinel structure have demonstrated advantages over the older refractory materials which comprise granular heat-resistant materials bonded in desired shapes with other heat resistant ceramic materials Because of their relatively dense spinel structure, co-fused cast refractories have been found to be substantially resistant to attack by corrosive melts such as glass and molten metal oxides. Many different compositions for co-fused cast refractories have been made and it has been found that certain compositions are superior to others for specific uses.
For example, co-fused chrome oxide-magnesia spinel grain in a chrome oxide matrix has been found to be generally effective in high temperature applications and environments that include slags containinq silicas, aluminas, and calcias. While generally effective in this particular application, the co-fused chrome oxide-magnesia is not free of problems. In severe cases a chemical reaction has been found to occur in which the magnesia of the chrome oxide-magnesia spinel leaves the spinel and combines with silica, alumina, or calcia, thereby weakening the refractory grain and resulting in cracking
The prior art believes that the spinel structure of the co-fused particles was the most stable form and does not recognize that this is not accurate, especially in view of the increased severity of conditions in which the refractories are currently expected to perform.
U.S. Pat. No. 4,490,474 discloses a co-fused cast ceramic composition comprising about 68-98 weight % Cr.sub.2 O.sub.3, 0.75-4 weight % MgO, and 1-34.25 weight % alumina. While containing a relatively high percentage of Cr.sub.2 O.sub.3, the co-fused composition also contains a high concentration of MgO along with a substantial amount of alumina. Thus it is not sufficiently resistant to attack for certain applications.
U.S. Pat. No. 3,773,531 discloses a refractory material with a high concentration of chromic oxide and no magnesia. However, the refractory is not a fused material, but rather a simple mixture of different materials cast into a block. Such a non-fused material lacks the dense fused structure necessary to resist severe corrosion.
U.S. Pat. No. 4,435,514 discloses a co-fused chromiamagnesia refractory product which contains a high percentage of magnesia, preferably in the range of 10 to 25% by weight.
U.S. Pat. No. 4,544,643 discloses a co-fused refractory product having low amounts of magnesia but also relatively low amounts of chromic oxide, i.e. no higher than 85% by weight. The composition also contains high amounts of alumina, i.e. 15-55 weight %. The alumina has been found to react with the silica of silica-containing slags and form spinels that degrade the refractory in a similar manner to the magnesia spinels discussed above.
U.S. Pat. Nos. 4,366,256, 3,759,728 and Japanese Patent 80045518 each discloses co-fused chromia refractory compositions containing relatively high amounts of magnesia.
Thus, the previous fused refractories have contained substantial amounts of oxides other than chromia in the belief that the co-fusion of chromia with the oxides was advantageous since it would cause chromia spinels to form which were believed to provide the most stable structures. It has now been discovered that the presence of these other oxides, especially magnesia, is detrimental to the performance of the resultant bodies in certain severe environments since the other oxides do not remain within the chromia spinels but rater are removed by reaction with the silica (and other oxides) and form new silica (and other) spinels, thereby weakening the original refractory structures.